1. Field
Exemplary embodiments of the present invention relate to a semiconductor design technology and, more particularly, to a semiconductor memory device which performs a refresh operation.
2. Description of the Related Art
A semiconductor memory device includes a plurality of memory banks for storing data, and each of the memory banks includes tens of millions of memory cells. Each of the memory cells includes a cell capacitor and a cell transistor, and the semiconductor memory device stores data through an operation of charging or discharging the cell capacitor. The electric charge stored in the cell capacitor must be ideally constant in the absence of any interruption. However, the electric charge stored in the cell capacitor changes due to a voltage difference from a surrounding circuit. That is, the electric charge may leak in a state where the cell capacitor is charged, or electric charge may be introduced in a state where the cell capacitor is discharged. As substantial change in the electric charge of the cell capacitor due to a leakage current, may result in data stored in the cell capacitor to be changed or lost.
For preventing such a data loss, a semiconductor memory device may perform a refresh operation.
With the development of process technology, integration of semiconductor memory devices has been gradually increasing. Increased integration has led to substantial reductions on the size of a memory bank. However, as the size of a memory bank becomes smaller, the intervals between memory cells and between neighboring word lines also become smaller. Recently, the interval between word lines has become so small that a coupling effect between adjacent word lines may occur causing neighboring memory cells to lose data stored therein.
For preventing such a data loss, existing semiconductor memory devices may refresh all memory cells of the memory bank more frequently. However, too many refresh operations may degrade the overall operation efficiency of the semiconductor memory device. Hence, different solutions are needed.